An object that NASA calls a 'tiny asteroid' explodes over Chelyabinsk, Russia, the largest such event since 1908. The shock wave shatters glass and rattles buildings, causing injuries.

The biggest piece of the space rock that broke apart over Russia to make it… (Associated Press )

MOSCOW — Without warning, a celestial object that NASA described as a "tiny asteroid" streaked above Russia's Ural Mountains early Friday before exploding, creating a shock wave that rattled buildings, shattered glass and injured hundreds of people.

Many witnesses in Chelyabinsk said they saw a white trail across the sky and a bright flash and heard a loud explosion seconds before buildings in the eastern part of the city were jolted.

Scientists said it was the largest such event in more than a century, since a blast that leveled 800 square miles of forest in 1908, the so-called Tunguska event, also in Siberia.

Its occurrence on Friday, hours before scientists were anticipating the close flyby of a larger asteroid, known as 2012 DA14, marked an extraordinary coincidence. Scientists said the two events were not related.

"When I saw some white narrow cloud moving outside the window, I ran up to it and saw a huge blinding flash," Nadezhda Golovko, deputy head of Chelyabinsk Secondary School No. 130, said in a phone interview.

"It was the way I would imagine a nuclear bomb. At first, there was no sound at all, as if I suddenly went deaf. Then I started hearing loud sounds of something exploding, four or five, one after another, and then the school windows started breaking," she said.

More than 1,100 people had been treated for injuries by late Friday, with about 50 hospitalized, Marina Moskvicheva, a spokeswoman for the Chelyabinsk regional health department, told Interfax.

U.S. scientists estimated that the object measured about 45 feet across, weighed about 10,000 tons and was traveling about 40,000 mph.

It exploded about 15 miles above the surface, causing a shock wave that triggered the global network of listening devices that was established to detect nuclear test explosions.

The force of the explosion was between 300 and 500 kilotons, equivalent to a modern nuclear bomb, according to Bill Cooke, head of the Meteoroid Environment Office at NASA's Marshall Space Flight Center in Huntsville, Ala.

NASA scientists believe the object originated from the asteroid belt, a vast collection of debris orbiting between Mars and Jupiter that consists of leftover bits from the formation of the solar system.

It probably traveled for a year before it burst into the atmosphere Friday.

Russian officials said the object, which they called a meteor, apparently entered the atmosphere over the north of Kazakhstan and flew over part of Russia before exploding over Chelyabinsk.

"We have deployed 28 stations in the area to monitor radiation levels, which up to now remain normal," said Vladimir Stepanov, chief of the Emergency Situations Ministry's crisis center.

He said that officials did not have sufficient time to issue a warning before the object entered the atmosphere.

The biggest piece to make it all the way to the ground was believed to have fallen into Chebarkul Lake about 60 miles west of Chelyabinsk, according to Russia-24, a government news television network. It showed video of a 24-foot-wide hole in the lake's thick ice.

Truck driver Andrei Chernov, who was heading to Chelyabinsk from nearby Kopeysk shortly after 9 a.m., said he saw what he thought was "a huge ball of fire silently rising all over Chelyabinsk."

"I thought some catastrophe was happening in the city," he said. "I started dialing my wife, but there was no connection."

Dozens of witnesses posted videos and photos to the Internet. Russia-24 aired images of the destruction at an indoor athletic stadium in Chelyabinsk, where at least two people were seen covered with blood among the debris. No one was killed, the report said.

Tatiana Borisevich, the science secretary of the Pulkovo Observatory in St. Petersburg, called the explosion over Chelyabinsk a unique study case for scientists. The last time scientists observed a similar-size object was in 2009 when one crashed in South Sudan. No remnants of it were ever retrieved, she said.

"Of course, we are sorry that so many people suffered from the explosion [Friday], but as scientists we are excited because now we have this unique study case using many existing videos of the incident to calculate its orbit to answer some questions we couldn't answer before," Borisevich said. "We also hope that now some parts of it can be found to determine its composition, whether it was stone, metal or ice."

Although as a rule big asteroids in space can be detected in advance, smaller objects can often enter Earth's atmosphere with little warning, Borisevich added.

The smaller asteroid was traveling in a very different trajectory and much faster than 2012 DA14, indicating they were not related, according to Paul Chodas, research scientist in the Near-Earth Object Program office at NASA's Jet Propulsion Laboratory in La Cañada Flintridge.